U.S. Energy Secretary Steven Chu accused Iran of nuclear "denial, deceit and evasion" on Monday, warning that Tehran's decision to move some uranium enrichment facilities to an underground bunker brings it closer to being able to producing the fissile core of a warhead.

But Iran's nuclear chief remained defiant. He announced that the subterranean Fordow facility criticized by Chu would likely start operating within six months and blamed Washington and its allies for Tehran's decision to redeploy beneath the earth -- a move, he said, was meant to protect its nuclear program from U.S. attack.

"The reasons we moved the Fordow site underground is that we want to make the Americans and their allies work tougher in order to destroy these facilities," said nuclear head Ferreydoun Abbasi Davani, who is also an Iranian vice president. He said the only reason already operating enrichment facilities elsewhere have not been attacked "is because we have kept them underground."

"The Americans are so afraid of their lives and their installations and facilities because of the al Qaida attacks," he said. "Why should they not give us the right in order to protect our installations? ... If they would not carry out so many devious actions we would establish our facilities above the ground."

Abbasi spoke to reporters after Chu criticized Iran's nuclear secrecy and refusal to heed U.N. Security Council demands to give up enrichment in comments that were notably outspoken for the head of an agency whose focus is on domestic energy, environmental and nuclear issues.

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"Iran has continued to engage in a long-standing pattern of denial, deceit, and evasion, in violation of its nonproliferation obligations," he told a 151-nation meeting of the International Atomic Energy Agency. "Time and time again, Iran has refused to satisfy legitimate concerns about the nature of its nuclear program -- selectively rejecting IAEA requests for access to, and information about, its nuclear facilities."

Iran recently announced that it would increase production of uranium enriched to a higher level than normally needed for reactor fuel and to do so at the hardened underground Fordow facility near the holy city of Qom. That has added to international worries because the path to making weapons grade uranium from higher enriched material is faster and easier than from the low-enriched uranium it has been producing for more than five years at its Natanz facility.

Chu referred to such concerns, telling delegates that "expanding, and moving underground, its enrichment to this level marks a significant provocation and brings Iran still closer to having the capability to produce weapons grade uranium.

"Pursuing this course raises serious questions over Iran's peaceful intent," he said, voicing concerns echoed by Britain and France at the conference.

The sharp tone of the exchanges on the opening day of the annual IAEA conference reflected the international divide over Iran's nuclear activities nine years after revelations that the Islamic Republic was secretly assembling a uranium enrichment facility at the central city of Natanz.

Since then, the U.N. Security Council has imposed four sets of sanctions on Iran for its refusal to stop enrichment. Tehran says it wants only to make fuel for a future reactor network but international intelligence suggesting it has worked -- or is still working -- on developing a nuclear warhead and other components of an arms program has sharpened fears that it will use enrichment for making fissile warhead material.

Chu's comments were particularly outspoken considering his department's main focus -- domestic energy, environmental and nuclear issues -- reflecting concerns that Iran might be seeking to develop nuclear weapons despite its denials of such aims.

"Iran has continued to engage in a long-standing pattern of denial, deceit, and evasion, in violation of its nonproliferation obligations," he told the meeting. "Time and time again, Iran has refused to satisfy legitimate concerns about the nature of its nuclear program -- selectively rejecting IAEA requests for access to, and information about, its nuclear facilities."

Abbasi's announcement that Fordow would start up later this year was sure to add to Western concerns.

"The machinery and the centrifuges are on the way to be installed," said. "We believe that within six months of time we are going to make ready the inauguration of the Fordow site."